Influence of grafting density on wall slip of a polymer melt on a polymer brush

We have investigated the shear flow of a polydimethylsiloxane (PDMS) melt on a PDMS brush of well-controlled density. As the shear rate, (gamma) over dot, is increased, we show the existence of a slip transition from a weak slip regime, where the extrapolation length of the velocity profile, b, is a constant b(0), to a strong one, where (gamma) over dot sticks to its valve at the transition (gamma) over dot(k). We discuss the influence of the grafting density Sigma on b(0) and (gamma) over dot *. We compare our results with a recent microscopic model.